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npr | We've all had the experience of watching a great athletic performance — from gymnast Mary Lou Retton defying gravity to Michael Jordan sinking a mind-blowing turnaround jumper — and wondered: Were they born with that talent or can you get there with hard work and practice?

Sports Illustrated senior writer David Epstein says scientists are learning a lot more about the role of genetics in athletic performance. In his new book, The Sports Gene, he looks at whether big league hitters have naturally faster reflexes, whether some people are born with speed and that delicate question of whether African-Americans are better athletes than whites. Epstein says that science now has answers, or at least insights, into all those questions. He joins Fresh Air's Dave Davies to talk about the secret to hitting a fastball and why slow dogs win the Iditarod.

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Interview Highlights

On the truth about baseball and softball hitter reflexes

"Going into it, I figured that they would have these superhuman reaction speeds because they face 100-mile-an-hour pitches everyday, and [softball pitcher] Jennie Finch's fastballs take exactly the amount of time as a mid-'90s baseball does. So the baseball comes at 60 feet 6 inches, 95 miles an hour; Jennie Finch throws from about 43 feet at about 65 miles an hour. Same exact time and the ball is bigger and yet they couldn't hit it all. It turns out that even the best hitters in the world have perfectly pedestrian reaction times."

On how, then, hitters manage to hit the ball

"They pick up on cues from the players' bodies before their pitch. So for a pitcher, without knowing it, the hitters are actually focusing in on the motion of the pitcher's shoulder and the pitcher's torso and hand. And then, as soon as the ball is released, [hitters focus] on what is called the flicker, which is a flashing pattern that the [ball's] red seams make as they rotate. And it's only picking up those anticipatory cues that allows the hitter to hit the ball.

"... This is a learned perceptual skill. And in fact, if you do a digital simulation, which some scientists have done, where you delete the pitcher's shoulder, [the Los Angeles Angels'] Albert Pujols becomes me, basically. You have to delete a little more than the shoulder to get him to that novice level, but he basically becomes a novice if you do that. And you can do the same thing with tennis players."

On breeding sled dogs for the Iditarod

"As I write in the book, it's not the fastest dogs that win. So sled dogs, when they were first bred for racing, the mushers bred for speed traits, and the idea was to race between checkpoints in the Iditarod very, very quickly, and they topped out in their top speed. And then what became popular because of [four-time Iditarod champion] Lance Mackey, who I write about, who couldn't afford to breed fast dogs, he had to breed instead dogs that were slower but would just go and go and go, and had a drive to pull the sled all the time and never wanted to stop. And it turns out — and scientists look at some of those sled dogs — they've actually been bred for motivation, they've been bred for work ethic. And the speeds of the Iditarod races are getting faster because the dogs are pulling longer, not faster."

On the question of whether African-Americans are predisposed for athletics

"Most of our ancestry as humans has occurred in Africa, so people have been in Africa for far longer than they've been outside of Africa. So genes for hundreds of thousands of years were evolving, changing inside of Africa, and then just a tiny group of people — maybe no more than 150 people, or a small group — left East Africa en route to populating the rest of the world. At each stop, their genes changed to accommodate their environments and sometimes just by random chance. ... But what this means is that most of the genetic differences that have been built up in our history are all still in Africa. All of us outside of Africa are just tiny subsets of a tiny subset that left Africa. So if you got rid of everyone in the world outside of Africa you would lose a little, but you would preserve most of the genetic variation for all of humanity.

"... [For] a particular trait, you might find the most diversity within an African population, as opposed to comparing someone in an African population and someone in a European population. So you might find the fastest 10 runners and the slowest 10 runners. But nobody is looking for the slowest 10 runners."